Jean,
Sorry. Your email arrived during my trip to Switzerland (I'm here for 3 months - so I'm still a bit jetlagged and getting settled in).
Here is an excerpt from one of our newsletters where we discuss estimation, that explains this:
There are various ways in which you can perform an estimate. Here are the most common types:
Estimation by expert judgment
Analogous Estimating
Parametric Estimating
Top-down Estimating
Bottom-up Estimating
The PMBOK Guide and the PM PrepCast both explain the concepts and approaches behind these types. For the exam you not only need to know these types but you also have to know the estimation ranges. And they are as follows: Order of Magnitude: -25% to +75%, a Budget Estimate is -10% to +25%, a Definite Estimate is -5% to +10% and a Final Estimate has a 0% range.
If you compare these numbers to the PMBOK Guide you will notice that there is a difference. The PMBOK Guide only mentions the ROM at -50% to +50% but not the other numbers. Don't be surprised. The PMBOK Guide only contains about 75% of the knowledge required for the PMP exam. The other 25% come from various other sources and the PMI does not specify exactly from where. The percentages that we have listed above are the generally accepted ranges and you can find them references both in many PMP prep books as well as on many free PMP websites. But don't be shocked if they are not in the PMBOK Guide.
And just to emphasize: When giving a quote to a customer, you will never give a single number (i.e. $10) you will always give a range ($8 - $12).
Overall I can say that these ranges are best described as "a headache". There is no authoritative source that has defined these ranges once and for all. All that PMP Exam students can do is to know the ranges from the PMBOK Guide and then learn the generally accepted ranges from other sources. Unfortunately (and there is nothing we can do about that) even the top PMP exam prep companies don't agree on the ranges. It's a headache and there is nothing that anyone can do and not even "Advil PM" will help...